The present invention relates to a secondary battery protection circuit for protecting a rechargeable secondary battery. The secondary battery protection circuit according to the present invention is used in a secondary battery unit having a secondary battery such as a lithium ion battery. It detects an over-charge state, over-discharge state, over-current state and the like of the secondary battery and performs a protecting operation in accordance with each of the states to protect the secondary battery.
There are various types of secondary batteries including nickel-cadmium batteries, nickel-hydrogen batteries, and lithium ion batteries. In order to charge the secondary battery, it is necessary to use a charger suitable for the type of the battery. Among the various types of secondary batteries, lithium ion batteries are vulnerable to over-charge and over-discharge. Therefore, it is essential that a secondary battery protection circuit of a battery unit having a lithium ion battery has a detection device for detecting an over-charge state and an over-discharge state of the lithium ion battery. Further, the secondary battery protection circuit must have a detection device for detecting an over-current state.
With respect to over-charge, when the lithium ion battery is charged with the charger, the battery voltage continues rising even in a full-charge state. When it enters the over-charge state, the battery can be broken due to an increase of the internal pressure of the battery, and shorting can occur between the electrodes because of deposition of lithium metal. For this reason, the lithium ion battery is charged at a rated current and a rated voltage. Further, a control voltage for charging at the rated voltage should not exceed the rating of the lithium ion battery. However, when the battery is charged with a charger for a secondary battery of a different type by mistake, the battery voltage may exceed the rated voltage of the lithium ion battery. An over-charge protecting function is a function of blocking the charging current such that the battery voltage will not exceed the maximum rating of the lithium ion battery in such a case.
With respect to over-discharge, the performance of a nickel-cadmium battery or nickel-hydrogen battery deteriorates unless it is charged after it is used until the battery capacity becomes zero. In other words, nickel-cadmium batteries and nickel-hydrogen batteries exhibit a so-called memory effect that is deterioration of performance attributable to insufficient discharging and charging repeated during the use of the batteries. On the contrary, lithium ion batteries are idealistic as secondary batteries in that they do not exhibit the memory effect. However, constituent substances of the lithium ion battery can deteriorate to reduce the life of the battery when the battery voltage falls below a predetermined value as a result of over-discharge. An over-discharge protecting function is a function of blocking the discharging current when the battery voltage falls below the predetermined value.
With respect to an over-current, a great current can flow when a positive terminal and a negative terminal of the lithium ion battery is erroneously shorted by some metal during storage or transportation of the lithium ion battery or when shorting occurs as a result of a failure of an apparatus to which the battery is connected. An over-current protecting function is a function of blocking the discharging current by detecting the value of the current with a secondary battery protection circuit.
The protecting operation of any of the above-described protecting functions is activated by detecting a voltage or current. Such a voltage or current may fluctuate for a very short time. In such a case, there is no need for the above-described protecting operation even in the case of a fluctuation with a great value. In order to achieve this, the secondary battery protection circuit includes a blind time setting circuit. The blind time setting circuit prevents, for example, the execution of the over-current protecting operation for a predetermined time or a blind time even when an over-current is detected. This applies to the over-charge and over-discharge operations.
However, as will be described in detail later, conventional secondary battery protection circuits have the following problems.
When the assembly of the secondary battery protection circuit is completed, a functional test is carried out on the same. For example, the function of an over-current detecting circuit is tested by repeatedly checking whether a protecting operation against the over-current properly takes place by applying an over-current actually. In this case, there is no need for the blind time, because the function of the over-current detecting circuit can be tested by only checking whether the protecting operation properly takes place against the over-current. However, since the blind time setting circuit also functions during the functional test, it is not possible to know whether the protecting operation properly takes place against the over-current unless the blind time set for the over-current passes. In addition, the above-described functional test must be carried out not only for the over-current but also for the over-charge and over-discharge. As a result, the secondary battery protection circuit as a whole has a waiting time attributable to the blind time that can not be ignored, which necessitates a long time to carry out the functional test.
It is therefore an object of the present invention to provide a secondary battery protection circuit having a blind time setting circuit in which a blind time can be reduced during a functional test.
It is a specific object of the present invention to provide a secondary battery protection circuit having a blind time setting circuit in which a blind time can be reduced even when the blind time setting circuit is manufactured by incorporating the same in an IC.
A secondary battery protection circuit according to the present invention detects at least one of an over-discharge state, over-charge state, and over-current state of a secondary battery and performs an operation in accordance with the detected state. The secondary battery protection circuit has a blind time setting circuit constituted by an IC for setting a blind time in accordance with the detected state. In an aspect of the present invention, the blind time setting circuit has a switching circuit for setting a blind time for a functional test that is shorter than a preset blind time.
Furthermore, according to the present invention, it is also provided a secondary battery unit which is provided with the above mentioned secondary battery protection circuit.